1. Field of the Invention
This invention is related to the field of plating and particularly to the field of quality control of electroplating baths.
2. Description of the Prior Art
Electroplating is a complex process involving many ingredients n the plating bath. The concentration of some of these ingredients must be kept within close tolerances in order to obtain high quality deposits. In some caes, chemical analysis of individual constituents can be made regularly (such as pH measurement for acid content), and additions made as required. However, other constituents such as leveling agents (brighteners) and impurities cannot be individually analyzed on an economical or timely basis by a commercial plating shop. Their concentration is low and their quantitative analysis is complicated and subject to error.
A prior art method of controlling the ingredients in an electroplating bath is simply to make regular additions of the particular ingredients based upon rules of thumb established by experience. However, depletion of particular ingredients is not constant with time or with bath use. Consequently, the concentration of the ingredient in the bath eventually diminishes or builds up out of tolerance.
A further prior art method of plating bath control is to plate articles or samples and visually evaluate the plating quality to determine if the bath is performing satisfactorily. In standard Hull cell and "bone pattern" tests, a specially shaped test specimen is plated and then evaluated to determine the quality of the deposit along the shape. These are time-consuming tests which give only rough approximations of the concentration of constituents in the bath.
The electroplating of through-hole interconnection in the manufacture of multilayer printed circuit boards is an example where high quality plating is required. It is known that the concentration of leveling agent, or brightener, must be maintained in the low ppm range in order to obtain acceptable deposits on printed circuit boards. The concentration of brightener fluctuates because of oxidation at the anode, reduction and inclusion at the cathode, and chemical degradation. When the brightener level is insufficient, deposits are burnt and powdery; whereas, excess brightener induces brittleness and non-uniform deposition ("fisheyes"). Hull cell tests, "bone pattern" tests, and tensile tests, combined with the periodic additions of fresh brightener, were, until the present invention, the only methods available to maintain a controlled brightener concentration. Since these methods are unreliable, circuit board quality suffers and rejection rates are high.